Nice idea, but I'd like if better if the rewards were based on total number of attack/assassinate/steal options rather than plunder as tab would give smaller builds who do not generate as much plunder the chance to win some of the higher rewards. If anyone else has already said this I'm sorry to have repeated you.
The players are put into clans based on their CS. So the rewards they earn will also be judged differently than players from other clans. Basically, smaller players will have to earn less plunder to get a reward than bigger players.